Borrelia burgdorferi, the etiological agent of Lyme disease, and Babesia microti, the etiological agent of human babesiosis, are transmitted by Ixodes dammini ticks, separately, as well as simultaneously, leading to concurrent infection in the host. We are proposing to study the development and transmission of each organism in larval and nymphal I. dammini, as well as the concurrent development of both organisms. The possibility of transmission of both organisms by adult ticks will be investigated experimentally. Hamsters, experimentally infected with primary isolates of B. burgdorferi and B. microti, will be the source of the organisms for the ticks. Development of both organisms within the tick will be studied by immunohistological techniques using monoclonal antibodies followed by biotinylated reagents as well as by nonspecific procedures. Quantification of B. burgdorferi in ticks will be done by measuring the uptake of 51Cr radiolabeled spirochetes injected into hamsters and by examination of cellular suspensions of ticks during the post engorgement period. Transmission potential of the subadult stages as well as the possibility of transovarial transmission will be studied by examination of ticks in the mammal hosts. These studies will yield highly specific reagents and new specific methodologies which will be valuable in the measurement of risk for these two diseases.